The invention relates to a device for positioning at least one offset plate on a positioning table of a pre-press equipment, comprising means to move the offset plate in a plane parallel to the positioning table to at least one reference point on the positioning table, in which said means act upon a working point situated in the supporting plane of the offset plate on the positioning table, in which said means act upon a working point situated in the supporting plane of the offset plate on the positioning table and wherein said means act upon said working point through an aperture in the positioning table.
For the positioning of at least one offset plate on a positioning table of a pre-press equipment, devices are known in which the offset plates are put on the positioning table by means of a loader in the vicinity of three roller shaped reference points. The offset plate is positioned with respect to these three rollers by means of three fingers which are placed facing the rollers and which push the offset plate against these rollers. Once there is contact between the offset plate and the three rollers, the plate is positioned and the plate is kept in its correct position by means of vacuum channels.
These devices has the disadvantage that different positioning tables with different equipment are necessary to position offset plates with different sizes. The positioning system is in that way dependent on the size of the offset plates.
A further disadvantage is that different means are necessary to perform the different movements to the reference points.
A solution to these disadvantages can be found in U.S. Pat. No. 4,790,584, in which a compliant linkage apparatus for providing, in a single stage, three degrees of freedom in a single plane. The apparatus comprises an upper plate, a lower plate under the upper plate, sliding means for allowing the upper and lower plates to slide relative to each other in X, Y, and theta, and orientation means for causing plates to attract each other in a particular orientation to resiliently resist relative lateral and rotational displacements of the plates. The device includes vacuum engagement members which are connected to a vacuum plate for engaging and disengaging the workpiece.
The problem with this apparatus is that it is not applicable on printing plates. The aluminium printing plates have thickness of 0.1 to 0.3 mm. Because of the fact that in this apparatus the workpiece is engaged and disengaged at its top surface through vacuum engagement members, when using printing plates, these printing plates will bend and it is very difficult to get them flat on the supporting surface. Furthermore, the upper layer of the printing plates consist of a very photosensitive emulsion layer, which will be damaged when sucking the printing plates at their top to the vacuum engagement members.
In DE 22 01 873, a device is described for positioning a printing plate which is susceptible for being held in a movable manner by means of a tightening strip, and provided with marking in relation to the cylindrical printing press plate holder which is arranged to receive the tightening strip of an offset press, in particular of a polychrome offset press. There are provided reference markings which, due to the correct position of the printing plate, will coincide with the markings of the printing plate, depending on which reference markings of the tightening strip can be brought into a defined position, in order to tighten the printing plate in a reproducible way in a relatively identical position in relation to the tightening strip. In the second embodiment of this patent application, an adjusting plate is provided which is slidably mounted against a ground plate through means of three micrometer screws. Between the adjusting plate and the ground plate a three-pointed support is provided. The adjusting plate is permanently remained in contact with the micrometer screws by means of a pre-tensioned spring which on the one hand is mounted to the adjusting plate, and on the other hand is mounted to the ground plate. In this device, the printing plate is moved together with the adjusting plate in relation to the ground plate by means of the micrometer screws.
The disadvantage of this device is first of all that a complex projector system is used to check when the reference markings coincide with the markings of the printing plate. Furthermore, also a complex positioning system for the printing plate is provided, using an extra adjusting plate to take care of the correct position of the printing plate. It is not the printing plate itself which is moved in order to make the reference markings coincide with the markings of the printing plate and consequently to bring the printing in a correct position.
The purpose of the invention is to provide in a device for positioning an offset plate on a positioning table of a pre-press equipment which doesn't show the abovementioned disadvantages.